Object mode scaling vs edit mode scaling for video game development
I don't think there are any significant efficiency implications, but I typically try to apply scale when making assets for the BGE, as some discrepancies with physics and collision bounds can occur and very large or small scales can cause precision issues.

For external engines I would apply scale just because of the exporting process (depending on the format, the scale may be applied automatically anyway)

It doesn't really matter whether you scale in edit or object mode as long as you apply it with &#9096; CtrlA> Scale when all is said and done.